In a radio communication system such as a wireless local area network (LAN) system, a digital cellular communication system, or the like, a plurality of radio stations shares a predetermined frequency band to perform communication. Thus, a signal which is received by a receiving station includes not only a signal (a desired signal) the destination of which is the receiving station but also a signal (an interfering signal) used for communication between radio stations which is irrelative to communication with the receiving station. These signals overlap with each other to generate a combined signal.
FIG. 78 illustrates an example of a radio communication system including a plurality of radio stations. The radio communication system in FIG. 78 includes a transmitting station 11, a receiving station 120, and interfering stations 13 and 14. The transmitting station 11 performs communication with the receiving station 120, and the interfering station 13 performs communication with the interfering station 14.
The transmitting station 11 converts into a radio signal 15 transmission data, the destination of which is the receiving station 120, and transmits the radio signal 15. The receiving station 120 receives and demodulates the radio signal 15 to obtain the transmission data from the transmitting station 11. By these operations, communication is performed between the transmitting station 11 and the receiving station 120.
On the other hand, the interfering station 13 transmits a radio signal 16, the destination of which is the interfering station 14, and the interfering station 14 receives it. Also, the interfering station 14 transmits a radio signal 17, the destination of which is the interfering station 13, and the interfering station 13 receives it.
Here, when a timing of transmitting the radio signal 15 overlaps with a timing of transmitting the radio signal 16 or 17, the receiving station 120 receives a combined signal into which the radio signal 16 or 17 overlaps with the radio signal 15, which is a desired signal.
In the case of receiving the signal into which the interfering signal overlaps with the desired signal, a probability that demodulation error of the desired signal occurs due to an effect of the interfering signal depends on an SIR (desired signal power to interfering signal power ratio) at the receiving station. For example, in the case where the transmitting timings of the transmitting station 11 and the interfering station 13 overlap with each other, if the distance between the interfering station 13 and the receiving station 120 is large in comparison with the distance between the transmitting station 11 and the receiving station 120 and if the received power of the interfering signal is sufficiently small at the receiving station 120 in comparison with the received power of the desired signal, the probability that demodulation error of the desired signal occurs is low. On the other hand, if the distance between the interfering station 13 and the receiving station 120 is small in comparison with the distance between the transmitting station 11 and the receiving station 120 and if the received power of the interfering signal is large at the receiving station 120 in comparison with the received power of the desired signal, the probability that demodulation error of the desired signal occurs is high.
The effect which the interfering signal has on the desired signal depends on channel frequencies of the desired signal and the interfering signal. In the case where the channel frequency of the radio signal 15 is the same as that of the radio signal 16 or 17, since the effect of the interfering signal is large, the probability that demodulation error of the desired signal occurs is high. On the other hand, in the case where the channel frequency of the radio signal 15 is different from that of the radio signal 16 or 17, the effect of the interfering signal is small. However, a radio signal is a broadband signal, and when a leakage power to outside the channel frequency band becomes large, such as the case where nonlinear distortion by a transmitting power amplifier occurs, or the like, the probability that demodulation error of the desired signal due to the effect of the interfering signal is high similarly as in the case of the same channel.
It is considered that the interfering station 13 and the interfering station 14 are included in a system different from that including the transmitting station 11 and the receiving station 120. For example, radio waves used by a wireless LAN system, a Bluetooth system, a cordless phone system, and the like are mixed in a 2.4 GHz band. Further, a microwave oven or the like which emits a leakage radio wave is not a radio station but exists as a generation source of a leakage radio wave, and such a leakage radio wave can be considered as an interfering signal. In a 5 GHz band, radio waves used by a wireless LAN system, a wireless access system, a radar, and the like are mixed.
In eliminating such an interfering signal from the received signal, the interfering signal is measured or presumed, and which interfering station the interfering signal overlapped with the desired signal is transmitted from is determined, thereby efficiently eliminating the interfering signal.
Patent Document 1 is provided as a conventional technique to eliminate the interfering signal from the received signal.
FIG. 79 illustrates a configuration of an interfering signal eliminator in the Patent Document 1. The interfering signal eliminator corresponds to the receiving station 120 in FIG. 78. The interfering signal eliminator comprises an interfering signal estimation section 201, an interfering signal extraction section 202, an adder 203, a memory 204, and a timing control section 205. The interfering signal eliminator assumes that a desired signal is a broadband signal while an interfering signal is a narrowband signal coming periodically. The interfering signal eliminator estimates and eliminates a narrowband signal (an interfering signal) which periodically overlaps with the desired signal. When the received power changes at constant intervals while the broadband signal is received, the interfering signal eliminator determines that the interfering signal overlaps with the broadband signal.
The interfering signal estimation section 201 estimates the interfering signal included in the received signal based on the received signal and the result of elimination of the interfering signal from the received signal. At this time, the interfering signal estimation section 201 uses a previous estimation result stored in the memory 204 as an initial value for an estimation value of this time, repeatedly performs calculation until the estimation value converges, thereby calculating a new estimation result. Interfering signal elimination means constituted of the interfering signal extraction section 202 and the adder 203 regards the estimation result calculated by the interfering signal estimation section 201 as a power level of the interfering signal, and eliminates the interfering signal from the received signal. Interfering signal estimation control means constituted of the memory 204 and the timing control section 205 stores the current estimation result of the interfering signal estimation section 201. The stored estimation result is used for estimation of the next interfering signal.
In the case where interfering signals come with a constant voltage and at known intervals as shown in FIG. 80, the interfering signal eliminator of the Patent Document 1 estimates the interfering signal based on a power difference between the received signal and the desired signal, and uses the estimation result for the next interfering signal estimation. Thus, the interfering signal eliminator can efficiently estimate and eliminate interfering signals which have constant packet lengths and come at a constant interval like a time division multiplex access (TDMA) signal.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2002-374179